It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended many different applications. However, the invention is especially useful when applied to a viscous fluid coupling or clutch which serves as a drive for an accessory of a vehicle engine, and will be described in connection therewith.
Viscous fluid couplings have received wide acceptance in the automobile industry, particularly for controlling the amount of torque transmitted to a vehicle radiator cooling fan. The most common form of such viscous fluid couplings is the air temperature response type such as illustrated in U.S. Pat. Nos. 3,055,473 to Oldberg et al and 3,809,197 to Clancey. In certain applications, however, it has become desirable to directly sense other system parameters, such as water temperature in the radiator, rather than the temperature of air passing through the radiator. To date, many arrangements have been proposed to accomplish this result. These arrangements, however, have made use of wet or dry plate clutches that are electrically actuated. In each of these arrangements, the advantages achieved through the use of viscous fluid have not been applied.
Another disadvantage common to viscous fluid couplings employing centrifugal pumps to circulate the fluid between a storage chamber and an operating chamber lies in the viscosity-temperature characteristic of typical silicon based viscous fluids. The viscosity of the fluid varies with ambient temperature and thus the transmission of torque between the shear surfaces can be controlled only by physical modulation of the circulating fluid flow between the operating and storage chambers. This flow is typically modulated by a valve element which selectively overlies a drain port interconnecting the storage and operating chambers. Although this approach has been acceptable for certain applications such as ON-OFF type fan drives, other applications demand more rapid or a tailored response of the clutch. Furthermore, the characteristically abrupt (dis)engagement of many known couplings renders them unsuitable for larger load applications.
More recently, so called electroviscous fluids have been developed for use in the fluid-actuated devices such as described in U.S. Pat. Nos. 4,782,927 and 4,744,914. Such fluids, when subjected to an electrostatic potential, become increasingly viscous or even solid. This effect, sometimes called the Winslow effect, is broadly described in Winslow's U.S. Pat. No. 2,417,850.
Electroviscous fluids exhibiting this property comprise a suspension of particles generally having a high dielectric constant in a fluid generally having a low dielectric constant. The particles are typically hydrophilic particles to which water is absorbed to impart a high dielectric property. In devices actuated with the use of an electroviscous fluid, the fluid is contained in a small gap between two electrically conductive members which serve as electrodes. When the device is a valve, the fluid can flow freely through this gap. When the device is a mechanical device, such as a clutch or brake, the two members can be moved freely relative to one another while maintaining the gap. When an electric potential is applied to the electrodes the fluid, in effect, solidifies thereby inhibiting fluid flow through a control orifice (in the case of the valve) or preventing relative movement between the electrodes (in the case of a brake or clutch).
The pressure which the device is capable of withstanding in the case of a valve or the force or torque which the device is capable of withstanding in the case of a clutch or brake is dependent upon the electrostatic potential applied between the electrodes. With an increasing electrostatic field, the shear force which the fluid is capable of withstanding is progressively increased until a point is reached when the fluid will no longer flow and effectively becomes solid. In the case of the valve, the reaching of this point means that the valve is fully closed, and in the case of a clutch or brake, the reaching of the solid state results in cessation of slipping.
A disadvantage of known electroviscous fluid-actuated devices employed as clutches or brakes, is that the fluid is permanently resident within the gap between the two torque transmitting members. Even with the fluid in its least viscous state, with no electrostatic field applied there across, the fluid continues to exhibit a finite degree of viscosity and thus continues to transmit a limited amount of torque across the members. In certain applications, it would be desirable to eliminate this efficiency robbing parasitic loss and resulting heat build-up and performance degradation. A further disadvantage of known electroviscous fluid actuated devices is in their relative large size and weight, shortcomings which are particularly disadvantageous for controlling vehicle accessories.